1. Field of the Invention
The present invention relates to refractory ceramic and metal materials and their method of manufacture, for use in filtering and sintering applications, and more specifically, for a method of controlling porosity in these materials.
2. Description of Related Art
High temperature gas phase exchange reactions are critical to the success of numerous technologies. For example, in the sintering of ceramic articles which contain volatile organic compounds, e.g., binders and plasticizers, the volatile organics must be thermally removed prior to the densification of the ceramic body. The binders and plasticizers added to the ceramic slurry are chosen so that they depolymerize into volatile hydrocarbons and carbonaceous residues upon heating to a particular temperature. This process, referred to as binder burnout, typically involves a polymer pyrolysis process and a carbon (char) oxidation process. The success of the binder burnout process is dependent upon the exchange of product and reactant gasses to and from the ceramic, and also within the ceramic itself. Binder removal can be a relatively simple process when conducted in a high oxygen content ambient environment, and when high gas exchange conditions are possible. However, under certain circumstances it may be necessary to perform the binder removal process under lower oxygen partial pressures, e.g., during co-sintering of ceramics and metals, or when it is necessary to fully or partially enclose the ceramic, thus reducing the gas exchange into and out of the ceramic. Under such conditions, the control of the product and reactant gas species to and from the ceramic body becomes critical.
The use of porous sintering fixtures, e.g., setter tiles, boxes, covers, and porous contact sheets, can greatly improve the kinetics of the gas exchange reactions. For ample gas exchange through porous fixtures, the fixtures should contain a high volume fraction of contiguous pores. The porous sintering fixtures allow for the exchange of product and reactant gas species into and out of ceramic bodies such as multilayer alumina, aluminum nitride, or glass ceramic substrates during the binder removal process. The more porous the sintering fixtures are, the better the gas species can diffuse through them to the actual substrate. The same is true for the contact sheets which may be applied to the surface of the multilayer ceramic substrate.
Important features pertaining to the porous material include total pore volume, pore size, connectivity of the pores, tortuousness of the porous network, and pore channel uniformity. In addition to the pore structure, since sintering of ceramics typically occurs at high temperatures, usually greater than 800.degree. C., the porous sintering fixtures must be able to withstand an extreme thermal environment without significant degradation in their mechanical properties or structure. Porous refractory materials, i.e., thermally stable materials, such as porous ceramics and metals, are typically used as sintering fixtures for these applications. Another common usage of this porous material is for filtering gases from the ambient.
However, introduction of pores in a ceramic material generally causes a decrease in strength of the resulting ceramic body. It has been observed that the decrease in strength of porous ceramic materials is exponential with porosity content. Yet, for porous ceramic and metal materials to be successfully utilized as sintering fixture materials or filter materials that enhance the high temperature gas exchanges, the porous materials must have a high volume fraction of contiguous pores with uniform pore geometry. With respect to filter materials, modulus of rupture or bend strength, thermal shock resistance, thermal expansion, modulus of elasticity, fracture toughness, thermal conductivity, hardness, density, and potential chemical reactivity are important criteria in the selection of a viable, durable filter. Although many of these material properties are available for dense ceramics, these ceramics have not been generated under conditions that control and regulate the porosity of the refractory ceramic or metal material.
Producing a durable, porous material with repeatable accuracy would enable the implementation of a consistent binder removal process during substrate production without concern for the variations in high temperature gas exchange kinetics that would otherwise result from the small differences in the porosity of the ceramic fixtures used in the sintering process. Consequently, it would be beneficial to those practicing in the art to have a process by which a controlled volume of contiguous pores, with a specific pore size, can be introduced into a refractory ceramic or refractory metal material.
In U.S. Pat. No. 4,777,153 issued to Sonuparlak et al., on Oct. 11, 1988, entitled "PROCESS FOR THE PRODUCTION OF POROUS CERAMICS USING DECOMPOSIBLE POLYMERIC MICROSHPERES AND THE RESULTANT PRODUCT", a colloidal suspension of polymeric microspheres of a selected size and shape are consolidated with aluminum oxide particles to form a compact. When the compact is heated, the microspheres are decomposed to leave pores. The resultant structure is then sintered to form a porous ceramic body with a plurality of pores, substantially the same size and shape, that are evenly distributed and noncontiguous throughout the ceramic material. However, this prior art teaches the addition of the polymeric microspheres to introduce noncontiguous (discrete) pores in the ceramic. In order to enhance the gas permeable characteristics of the material the pores must be contiguous (interconnected) to be effective in sintering and/or filtering applications.
A common method to introduce a contiguous pore structure into a ceramic is by partially sintering the ceramic. For aluminum oxide ceramics, this process is usually performed at a temperature of 1000.degree. C. to 1350.degree. C. for 12 to 24 hours after a preliminary heating at a lower temperature for removal of the binder. In the case of setter tiles, the porosity is desirable since it allows the ambient gases to flow through the setter tile and contact the ceramic substrate laminate, thereby rendering more efficient the removal of the binder decomposition products.
In U.S. Pat. No. 5,045,511 issued to Bosomworth et al., on Sep. 3, 1991, entitled "CERAMIC BODIES FORMED FROM YTTRIA STABILIZED ZIRCONIA-ALUMINA", ceramic bodies used in the filtration of molten metal are formed by immersing a porous organic substrate material in an aqueous thixotropic slurry and then thermally removing the organic and sintering the ceramic material. This results in a reticulated ceramic after sintering. However, this prior art neither discloses or suggests the mixing of organic powders with ceramic powders to form a porous ceramic, and then use the controlled porosity of the ceramic specifically for high temperature gas phase reaction control.
In U.S. Pat. No. 5,563,212 issued to Dismukes et al. on Oct. 8, 1996, entitled "SYNTHESIS OF MICROPOROUS CERAMICS", microporous ceramic compositions are prepared by first forming an intimate mixture of a ceramic precursor with additive particles to provide a composite intermediate, followed by pyrolysis of the composite intermediate under an inert atmosphere in sequential stages. Here, in the sintering process the ceramic is initially sintered under reducing conditions, i.e., using only inert ambient conditions. The ability to use oxidizing ambient conditions is neither disclosed or suggested. Also, although the addition of a fugitive phase to produce a porous material is paramount to this prior art, there is no suggestion to control the performance of the material, i.e., pore volume, connectivity of the resulting pores, and most importantly, gas exchange characteristics of the porous ceramic.
The disclosure of all of the above references is incorporated by reference herein.
By controlling the time and temperature used in the sintering process, the amount and connectivity of the pores can be controlled to a certain degree. However, as the time and temperature are increased, the amount of pores and the connectivity of the pores decreases. In addition, since incomplete sintering of the ceramic is necessary to achieve the proper pore structure, in most instances, the resulting porous ceramics have insufficient mechanical properties as a result of poorly sintered ceramic particles in the ceramic network. Also, since the sintering of the ceramic is very sensitive to the initial ceramic particle size, sintering temperature, and hold times, it is difficult to produce a consistent porous ceramic article. This would require the sintering process being interrupted at the same pore volume and pore structure in order to achieve the same final properties of the porous ceramic. Ideally, the introduction of pores into a ceramic or refractory metal material by a method which allows for independent control of pore volume and pore size, with or without the use of partial sintering of the final porous material, would greatly enhance the current state of the art.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for controlling the porosity within ceramic or refractory metal materials used in filtering and gas permeation applications.
It is another object of the present invention to provide a durable porous material with repeatable accuracy.
A further object of the present invention is to provide a porous material having a high volume fraction of contiguous pores with uniform pore geometry.
Yet another object of the present invention is to greatly improve the kinetics of the gas exchange reactions.
Another object of the present invention is to provide a method for controlling porosity of a gas permeable material during the thermal decomposition of the fugitive phase.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.